Laboratory rearing of Anopheles arabiensis: impact on genetic variability and implications for Sterile Insect Technique (SIT) based mosquito control in northern Sudan

Defining the genetics of the widely used G3 strain of the mosquito, Anopheles gambiae

Mosquito species in the Anopheles gambiae complex have been referred to as "the deadliest animals in the world" due to their role as vectors of malaria throughout sub-Saharan Africa. Consequently, An. gambiae was among the first species to have its whole genome sequenced in 2002 and it continues to be the subject of intense study. An. gambiae is one member of a nine member species complex and, along with its sister species, An. coluzzii, is among the most important vectors of human malaria. Laboratory research on malaria vectors across a broad range of disciplines utilizes a strain known as G3, which was established in 1975 from mosquitoes collected from McCarthy Island, The Gambia. This strain is well known to be a mongrel strain, nonetheless it is often referred to as An. gambiae, which it is not. The issue with G3 goes far beyond the typical inbreeding associated with long-standing laboratory colonies. G3 is an An. gambiae/An. coluzzii interspecific hybrid. Although these two species are known to hybridize in nature, the pattern of interspecific introgression in G3 we describe in this paper is unlike any observed in natural populations. In this report we provide an in-depth analysis of the genetics of the G3 strain and compare it with natural populations of its two parental species. We discuss potential concerns that results obtained from research using the G3 strain may not apply to populations of these mosquito species as they occur in nature.

Impact of long-term mass-rearing on the genetic structure of tsetse fly Glossina palpalis gambiensis colonies

Tsetse flies are the sole cyclic vectors of African trypanosomes, which cause human and animal African trypanosomiases in Africa. Tsetse fly control remains a promising option for disease management. The sterile insect technique (SIT) stands as an environmentally friendly tool to control tsetse populations. SIT requires the mass-rearing of competent sterile males to mate with wild females. However, long-term colonization might affect the genetic structure of the reared flies. This study investigated the genetic structure of four Glossina palpalis gambiensis colonies of different ages: two originating from Senegal (SEN and ICIRSEN) and two from Burkina Faso (CIR and IBD). Samples from these colonies were genotyped at ten microsatellite loci, followed by downstream population genetic analyses. The results show that the two colonies from Burkina Faso collected from close sites (∼20 km apart) over 45-year interval retained the same genetic background (FST_CIR∼IBD ≈ 0, P-value = 0.47). These flies were however, genetically different from those from the Senegal colonies (FST_CIR∼SEN ≈ 0.047; FST_IBD∼SEN ≈ 0.058, P-value = 10-4). Moreover, no significant difference was detected in the gene diversity of the CIR and IBD colonies, with HS values of 0.650 and 0.665, respectively. The inbreeding coefficient showed that all four colonies where under Hardy-Weinberg equilibrium, with FIS values of 0.026, 0.012, -0.064, and 0.001, for CIR, IBD, ICIRSEN, and SEN, respectively. Furthermore, no sign of a recent bottleneck was identified in tsetse samples from any of the four colonies. The results suggest that long-term mass-rearing of tsetse flies has no significant impact on their genetic background and diversity.

Passive Outdoor Host Seeking Device (POHD): Designing and Evaluation against Outdoor Biting Malaria Vectors

Odor-baited devices are increasingly needed to compliment long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) for control of residual malaria transmission. However, the odor-baited devices developed so far are bulky, dependent on the source of electricity and carbon dioxide (CO2), and they are logistically unsuitable for scaling up in surveillance and control of malaria vectors. We designed a passive and portable outdoor host seeking device (POHD) and preliminarily evaluated suitable components against Anopheles arabiensis that maintains residual malaria transmission. Experiments were conducted using semifield reared An. arabiensis within the semifield system at Ifakara Health Institute (IHI) in southeastern Tanzania. These mosquitoes were exposed to Suna traps® baited with BG lures or source of light and augmented with carbon dioxide (CO2) in view of identifying best attractants necessary to improve attractiveness of designed POHD. Two Suna traps® were hanged at the corner but outside the experimental hut in a diagonal line and rotated between four corners to control for the effect of position and wind direction on mosquito catches. Furthermore, mosquitoes were also exposed to either a bendiocarb-treated or bendiocarb-untreated POHD baited with Mbita blend, Ifakara blend, and worn socks and augmented with warmth (i.e., 1.5 liter bottle of warm water) inside an experimental hut or a screened rectangular box. This study demonstrated that mosquitoes were more strongly attracted to Suna trap® baited with BG lures and CO2 relative to those traps baited with a source of light and CO2. The POHD baited with synthetic blends attracted and killed greater proportion of An. arabiensis compared with POHD baited with worn socks. Efficacy of the POHD was unaffected by source of warmth, and it was reduced by about 50% when the device was tested inside a screened rectangular box relative to closed experimental hut. Overall, this study demonstrates that the POHD baited with synthetic blends (Mbita and Ifakara blends) and bendiocarb can effectively attract and kill outdoor biting malaria vector species. Such POHD baited with synthetic blends may require the source of CO2 to enhance attractiveness to mosquitoes. Further trials are, therefore, ongoing to evaluate attractiveness of improved design of POHD baited with slow-release formulation of synthetic blends and sustainable source of CO2 to malaria vectors under semifield and natural environments.

Plant extracts for developing mosquito larvicides: From laboratory to the field, with insights on the modes of action

In the last decades, major research efforts have been done to investigate the insecticidal activity of plant-based products against mosquitoes. This is a modern and timely challenge in parasitology, aimed to reduce the frequent overuse of synthetic pesticides boosting resistance development in mosquitoes and causing serious threats to human health and environment. This review covers the huge amount of literature available on plant extracts tested as mosquito larvicides, particularly aqueous and alcoholic ones, due to their easy formulation in water without using surfactants. We analysed results obtained on more than 400 plant species, outlining that 29 of them have outstanding larvicidal activity (i.e., LC₅₀ values below 10 ppm) against major vectors belonging to the genera Anopheles, Aedes and Culex, among others. Furthermore, synergistic and antagonistic effects between plant extracts and conventional pesticides, as well as among selected plant extracts are discussed. The efficacy of pure compounds isolated from the most effective plant extracts and - when available - their mechanism of action, as well as the impact on non-target species, is also covered. These belong to the following class of secondary metabolites: alkaloids, alkamides, sesquiterpenes, triterpenes, sterols, flavonoids, coumarins, anthraquinones, xanthones, acetogenonins and aliphatics. Their mode of action on mosquito larvae ranges from neurotoxic effects to inhibition of detoxificant enzymes and larval development and/or midugut damages. In the final section, current drawbacks as well as key challenges for future research, including technologies to synergize efficacy and improve stability - thus field performances - of the selected plant extracts, are outlined. Unfortunately, despite the huge amount of laboratory evidences about their efficacy, only a limited number of studies was aimed to validate their efficacy in the field, nor the epidemiological impact potentially arising from these vector control operations has been assessed. This strongly limits the development of commercial mosquito larvicides of botanical origin, at variance with plant-borne products developed in the latest decades to kill or repel other key arthropod species of medical and veterinary importance (e.g., ticks and lice), as well as mosquito adults. Further research on these issues is urgently needed.

A review on the progress of sex-separation techniques for sterile insect technique applications against Anopheles arabiensis

The feasibility of the sterile insect technique (SIT) as a malaria vector control strategy against Anopheles arabiensis has been under investigation over the past decade. One of the critical steps required for the application of this technique to mosquito control is the availability of an efficient and effective sex-separation system. Sex-separation systems eliminate female mosquitoes from the production line prior to irradiation and field release of sterile males. This is necessary because female mosquitoes can transmit pathogens such as malaria and, therefore, their release must be prevented. Sex separation also increases the efficiency of an SIT programme. Various sex-separation strategies have been explored including the exploitation of developmental and behavioural differences between male and female mosquitoes, and genetic approaches. Most of these are however species-specific and are not indicated for the major African malaria vectors such as An. arabiensis. As there is currently no reliable sex-separation method for An. arabiensis, various strategies were explored in an attempt to develop a robust system that can be applied on a mass-rearing scale. The progress and challenges faced during the development of a sexing system for future pilot and/or large-scale SIT release programmes against An. arabiensis are reviewed here. Three methods of sex separation were examined. The first is the use of pupal size for gender prediction. The second is the elimination of blood-feeding adult females through the addition of an endectocide to a blood meal source. The third is the establishment of a genetic sexing strain (GSS) carrying an insecticide resistance selectable marker (dieldrin-resistance rdl gene and/or other GABA receptor antagonists that can be used as alternative insecticides to dieldrin) or a temperature-sensitive lethal marker.

A low-cost, battery-powered acoustic trap for surveilling male Aedes aegypti during rear-and-release operations

The Aedes aegypti mosquito is a primary vector of several serious arboviruses throughout the world and is therefore of great concern to many public health organizations. With vector control methodology pivoting towards rearing and releasing large numbers of genetically modified, sterilized, or Wolbachia-infected male mosquitoes to control vector populations, economical surveillance methods for release tracking becomes increasingly necessary. Previous work has identified that male Ae. aegypti are attracted to female wingbeat frequencies and can be captured through artificial playback of these frequencies, but the tested systems are cost-prohibitive for wide-scale monitoring. Thus, we have developed a simple, low-cost, battery-powered, microcontroller-based sound lure which mimics the wingbeat frequency of female Ae. aegypti, thereby attracting males. We then tested the efficacy of this lure in combination with a passive (non-powered) gravid Aedes trap (GAT) against the current gold-standard, the Biogents Sentinel (BGS) trap, which requires main power (household power) and costs several times what the GAT does. Capture rates of male Ae. aegypti in sound-baited GATs (Sound-GATs) in these field tests were comparable to that of the BGS with no inhibitory effects of sound playback on female capture. We conclude that the Sound-GAT is an effective replacement of the costly BGS for surveillance of male Ae. aegypti mosquitoes, particularly in the developing countries where funding is limited, and has the potential to be adapted to target males of other medically important species.

A highly stable blood meal alternative for rearing Aedes and Anopheles mosquitoes

We investigated alternatives to whole blood for blood feeding of mosquitoes with a focus on improved stability and compatibility with mass rearing programs. In contrast to whole blood, an artificial blood diet of ATP-supplemented plasma was effective in maintaining mosquito populations and was compatible with storage for extended periods refrigerated, frozen, and as a lyophilized powder. The plasma ATP diet supported rearing of both Anopheles and Aedes mosquitoes. It was also effective in rearing Wolbachia-infected Aedes mosquitoes, suggesting compatibility with vector control efforts.